Zoologger: How to eat without a digestive system

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Parastichopus californicus
Habitat: Shallow seas along the west coast of the US

On the sandy sea floor, two spiny goliaths are in a face off. In one corner is the giant California sea cucumber, a formidable organism at half a metre long; in the other is the sunflower seastar. Don't be fooled by its endearing name: this guy is twice the size of the sea cucumber. That, however, does not make it a shoe-in for victory. The sea cucumber has an unusual trick up its sleeve.

Having failed to escape its foe, the sea cucumber goes for the explosive option. It squirts its digestive system out of its anus in a tangled, sticky mess, confusing the seastar and allowing it to get away.

The sea cucumber has escaped being eaten, but now it has a new problem. Surely, without a digestive system, it can't eat anything? Think again. This animal can eat through its anus.

Mop of tentacles

Sea cucumbers may look like slugs, but they are actually echinoderms, related to starfish and sea urchins. The giant California sea cucumber is a scavenger that prowls the sea floor. Its head has an array of mop-like tentacles, which it splays onto the sediment. The tentacles pick up tiny chunks of food, and it then inserts them into its mouth to feed.

Like many sea cucumbers, it can eviscerate itself when under attack or otherwise stressed. Many animals can shed limbs, tails or skin when under attack, a process called autotomy ? and sea cucumbers simply take it further than most. It seems the giant California sea cucumber can even break down and reabsorb most of its organs during winter, when food is scarce, and become dormant.

Its anatomy is, by human standards, a little peculiar. It doesn't have gills or lungs, but it does have a respiratory system that is directly connected to its anus, through which it pumps in water. This flows into a network of branched tubes called the respiratory tree, which takes in oxygen from the seawater.

But the respiratory tree can take in more than oxygen, says Richard Strathmann of the University of Washington in Friday Harbor ? - it also takes up dissolved organic compounds.

That may seem a bizarre way to eat, but it's perhaps not too surprising. Most echinoderms can absorb dissolved food through exposed skin, so extending that function to the "lungs" is not too great a leap. However, working with William Jaeckle of Illinois Wesleyan University in Bloomington, Strathmann has shown that the giant California sea cucumber can go much further.

Bottom feeder

Strathmann and Jaeckle gave captive sea cucumbers two kinds of labelled food. Some got single-celled algae, inoculated with radioactive carbon-14; others received polysaccharides or proteins that contained iron.

At intervals over the next 26 hours, the researchers took samples from different tissues within the sea cucumbers, and tested them for carbon-14 and iron. Most of the carbon-14 wound up in the respiratory tree, with very little in the digestive system, suggesting it was being taken in through the anus and respiratory system. The same pattern was seen with iron.

That means the sea cucumber isn't limited to absorbing dissolved nutrients via its anus and respiratory tree ? it can do take in food such as algae, too. It is the first animal known to do so, Strathmann says.

It's not clear how the respiratory tree digests the algae. Nor is it clear why the anal feeding evolved. Strathmann says it could well be a backup to allow the sea cucumber to feed without its main digestive system. Alternatively, by allowing it to feed on algae as well as rubbish on the seabed, it could be a way to take in a range of nutrients. Other animals, in particular simple worms, may also have evolved anal feeding, and for similar reasons.

Strathmann says the sea cucumber probably uses its anal feeding as a supplement, and still gets the majority of its food through its mouth and digestive system. "But even as a dietary supplement, nutrition taken through the anus is a remarkable reversal of what we expect."

Journal reference: Invertebrate Biology, doi.org/kcm

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